Field of the Invention
The present invention relates to an image display apparatus and an electronic apparatus having a projector function each synthesizing a plurality of laser beams of different color components to project a color image.
Description of the Background Art
Various image display apparatuses, such as a laser projector, each synthesizing laser beams of a red color component (R), a green color component (G), and a blue color component (B) to project and display a color image on a projection surface are put to practical use. Such an image display apparatus must create the color state of a beam obtained by synthesizing laser beams of R, G, and B in order to project and display a color image with high color reproducibility.
A laser diode (LD) employed as a laser beam source for each color has such characteristics that the output of a laser beam is started when a driving current reaches at least a threshold current and thereafter the light output (the light quantity of the laser beam) is increased with an increase in the driving current. Thus, the LD is a light-emitting element whose light output is varied according to the amount of supply of the driving current, but as another factor in varying the light output, there is a temperature change. This is because the threshold current of the LD is changed according to the temperature, and the threshold current is changed due to heat generation of the LD itself, so that the light output is varied. This causes loss of a white balance.
The relationship between the driving current, the threshold current, and the light output is described with reference to FIG. 7. In view (a) of FIG. 7, the driving current is initially less than the threshold current (is zero, for example), thereafter a certain amount of driving current equal to or larger than the threshold current is continuously supplied in a period Ta, and thereafter a certain amount of driving current lower than the driving current in the period Ta and equal to or larger than the threshold current is continuously supplied in a period Tb. When the driving current in this pattern is supplied to the LD, as shown in view (b) and (c) of FIG. 7, the threshold current of the LD is increased with time and the light output is gradually reduced with this and is stabilized at a value less than a target value in the period Ta. In the later period Tb, the threshold current of the LD is reduced with time, and the light output is gradually increased with this and is stabilized at a value less than a target value similarly to in the period Ta.
As a technology to inhibit this variation in the light output, there is APC (automatic power control). In APC, for example, the light output (the light quantity of the laser beam) from the LD is detected and is compared with the target value, and the amount of supply of the driving current is so controlled that the detection value approaches the target value, whereby a variation in the light output is inhibited.
Japanese Patent Laying-Open No. 9-058048 (1997) discloses a technology to measure light output variation characteristics when a light source is on and off, obtain a decay rate and a recovery rate from the measurement result, and determine correction data of an (m+1)th pixel from image density data of the (m+1)th pixel and accumulated data to an m-th pixel. In this technology, no consideration is given to the case where the decay rate converges.
Japanese Patent Laying-Open No. 59-163950 (1984) discloses a technology to measure time change characteristics of light intensity and correct current to cancel the amount of change of the light intensity at the time point at which a modulation signal is input. This technology deals with only the reduction of an output, and no consideration is given to the recovery of the output.
Japanese Patent Laying-Open No. 2002-254697 discloses a technology to detect a light output variation before and after a line, superimpose a current when the light output is varied by a certain amount or more, and prevent superimposition of a current when the light output is varied by less than a certain amount. In this technology, processing is performed assuming a variation in image data as an output variation identical to that in the case where an apparatus operates with a constant current.
When the APC is performed each time one line in a horizontal direction (direction X) is scanned, for example, light unnecessary for a user is frequently emitted. Furthermore, when the APC is performed for each frame period in order to inhibit emission of the unnecessary light as much as possible, for example, a blanking period during which the light output from the LD is off is generated, and hence it is necessary to consider an increase of the light output after the blanking period.